1. Field of the Invention
This invention relates to a downhole valve system, and in its preferred embodiment, to a well casing shut-in valve system. The preferred valve system of the invention relates to a downhole flapper valve assembly useful for controlling the flow of hydrocarbons and drilling fluids upwardly through a well casing whenever the drill string is tripped during drilling, and a shift head assembly adapted to engage a flow sleeve within the subject flapper valve assembly to selectively open and close the flapper valve.
2. Description of the Prior Art
During well drilling operations, the drill string is frequently tripped from the well for bit or motor replacement. Problems can arise during tripping if hydrocarbon fluids from downhole formations overpressure the drilling fluid in the well bore. Such problems are particularly likely to occur in highly deviated wells, including but not limited to so-called "horizontal drilling" operations in which the well bore descends vertically to a desired kick-off point, and then veers off horizontally through numerous vertical fractures. When it becomes necessary to come out of the hole and the bottom hole assembly is drawn up into the vertical casing, the well may begin to produce.
One technique previously used to control the upward flow of hydrocarbon fluids when wells become underbalanced during tripping has been to seal off the well bore at the wellhead and to condition the drilling mud by increasing its weight sufficiently to overpressure the hydrocarbon flow. As the drill string is withdrawn to shallower depths, this mud conditioning procedure may have to be repeated several times, progressively increasing the weight of the mud and also increasing costs. To avoid the need for repeatedly reconditioning the drilling fluid during tripping, a valve assembly can be installed downhole to serve as a check valve on the upward flow of hydrocarbon fluids.
The use of flapper valves as check valves or safety valves in subterranean wells has previously been disclosed, for example, in U.S. Pat. Nos. 2,447,842; 4,531,587; 4,706,933; 4,926,945 and 4,977,957.
U.S. Pat. No. 2,447,842 discloses the use of a flapper valve as a back-pressure check valve inside the drill stem, preferably near the bottom of the stem just behind the drill collar of the bit. The actuating member of the valve is a tubular piston with a flared head that moves upwardly under back-pressure from within the well to close the flapper upon withdrawal of a tool string.
U.S. Pat. No. 4,531,587 discloses a downhole flapper valve and actuator in which the downwardly facing surfaces of the actuating sleeve are shaped to cooperate with the upwardly facing cylindrical segment surfaces of the flapper valve body so that the effective point of application of the downward force produced by the actuating sleeve has a roughly equivalent moment arm as the resultant upward force produced by downhole fluid pressure. This construction is said to minimize any torsional moment being applied to the flapper which must be absorbed by the hinge pin. A pair of helical springs apply a torsional bias to the ends of integral hinge pins to urge the flapper valve to its closed position.
U.S. Pat. No. 4,706,933 discloses a flapper valve having a control link attached to the flapper at a centralized point removed from the hinge. The control structure comprises a hydraulically actuated piston moveable in a valve control cylinder. The piston is biased by a compression spring in the direction of the valve. A damping structure is provided to control the movement of the piston in its cylinder.
U.S. Pat. No. 4,926,945 discloses a subsurface well safety valve having a curved flapper with a concave surface which forms a sealing surface with a valve seat having a coacting contoured seating surface A flow tube is telescopically moveable in the housing for controlling the movement of the valve closure member, and the lower end of the flow tube is a cylindrical surface having a radius substantially equal to the radius of the concave sealing surface. Hydraulic piston and cylinder means are provided in the housing for actuating the flow tube.
U.S. Pat. No. 4,977,957 discloses a subsurface well safety valve having a flapper valve, a telescopically moveable, lightweight flow tube for controlling the movement of the valve closure member, and a hydraulic piston and cylinder means for actuating the flow tube.
The flapper valves in each of the prior art patents disclosed above opens and closes by rotating through an angle of 90 degrees. The large differential pressures and high fluid velocities sometimes encountered during use of such valves in downhole applications can cause stress concentrations leading to valve failure due to broken springs, hinges, or the like. Others have previously sought to reduce the likelihood of such failures by using control pistons in combination with compression springs or hydraulic cylinders to dampen the forces exerted against flappers and their hinge points, or by constructing valve parts from lighter weight materials.